1. Field of the Invention
This invention relates to a fuel vapor capturing canister capturing fuel vapor produced in a fuel system of an engine by means of a layer of adsorbent material to prevent dissipation of the fuel vapor into the atmosphere, and more particularly to such a canister of the type having a plurality of divided layers of the adsorbent material.
2. Description of the Prior Art
Japanese Patent Application Kokai No . 50-22921 (unexamined published patent application) discloses a conventional canister of the above-described type. Referring to FIG. 7 illustrating the disclosed canister, a cylindrical casing 1 formed of an impermeable material accommodates an activated carbon layer 2 therein. One of two surfaces of the activated carbon layer 2 communicates with the atmosphere through an opening 3a of a retainer. The other surface of the activated carbon layer 2 communicates both with a fuel tank 4 and with an intake pipe 5 through separate openings 1a and 1b respectively. The activated carbon layer 2 is divided into two layers 2a and 2b axially of the casing 1 by permeable plate-shaped elastic body 6.
In the above-described construction, fuel vapor produced in the fuel tank 4 enters the upper activated carbon layer 2a through the opening 1a. Passing through the permeable elastic body 6, the fuel vapor then enters the lower activated carbon layer 2b. The fuel component is thus captured by the activated carbon as the fuel vapor passes through its layer. Upon the start of an engine, the negative pressure is supplied to the opening 1b communicating with the intake pipe 5. Air is drawn through the lower activated carbon layer 2b, the elastic body 6 and the upper activated carbon layer 2a sequentially, whereupon the fuel component captured by the activated carbon is purged therefrom to be supplied through the intake pipe 5 to the engine.
The inventors have found that the fuel vapor capturing and purging efficiency can be improved when the path of the fuel vapor passing through the adsorbent material is lengthened. In the above-described canister, however, the air containing the fuel vapor passes through the elastic body 6 when flowing from the activated carbon layer 2a to the activated carbon layer 2b and vice versa. When the thickness of the elastic body 6 is ignored, the distance that the air containing the fuel vapor passes through the activated carbon layer 2 depends upon the length of the casing 1 . The fuel vapor capturing efficiency is lowered as the casing 1 becomes shorter. Thus, the above-described canister poses a problem of the fuel vapor capturing efficiency.